Aluminum alloy system



United States Patent 3,104,189 ALUMINUM ALLOY SYSTEM Nicholas Alton Wagner, Chester, Va., assignor to Reynolds Metals Company, Richmond, Va, a corporation of Delaware No Drawing. Filed Oct. 17, 1960, Ser. No. 62,835 3 Claims. (Cl. 14832.5)

This invention relates to new heat treatable wrought aluminum alloys containing magnesium and silicon as essential components, which are capable of air quenching while being extruded and of developing high mechanical properties when subsequently artifically aged. More particularly, the invention concerns such heat treated alloys which possess a combination of mechanical strength, a high degree of electrical conductivity, a high degree of surface luster and good extrudibility or forming. This application is a continuation-in-part of my application Serial No. 843,636, filed October 1, 1959.

Heat treatable wrought aluminum alloys in which the major portion of alloying elements consists of magnesium and silicon, whch are present principally in the form of the intermetallic compound magnesium silicide Mg Si, are well known. The most commonly used of these alloys are Nos. 6061, 6062 and 6063. The percentage composition limits of .these alloys are as follows:

TABLE I i 6061 i 6062 i 6063 Silicon 0.40-08 GAO-0.8.".-. 0.200 6 0n 0.7 max. 0.7 max" Copper 0.15-0.40..." 0.15-0.40". 'M'wne mm 0.84 2 0.8-1.2 0 45-09 Manganese 0.15 max.. 0.15 max 0 10 max Chromium 0.15-0 004-0 14 0 max Zinc 0.25 max 0.25 max 0 10 max Titanium 0.15 max 0.15 max. 0 10 max Others, each 0.05 max 0.05 max 0 05 max Others, total 0.15 max. 0.15 max 0 15 max Balance Al Al A1 While these known magnesium-silicide containing alloys are widely .used and possess good formability, weldability, corrosion resistance and mechanical strength after heat treatment, alloys 6061 and 6062 exhibit relatively poor surface finish and brightness after extrusion and water quenching to the T-4 temper condition. Alloy 6063 possesses a bright surface which is substantially free of pick up of aluminum or aluminum oxide which occurs as the metal leaves the die, as well as other surface irregularity. However, its maximum strength does not achieve that of the other alloys mentioned.

A demand has existed increasingly for higher strength aluminum alloy extrusions, such as, for example, those used for furniture and trailers, having strength properties in the 6061 and 6062 range, but which would also possess surface brightness approaching or equal to that obtainable with the 6063 type alloy, and optionally which would also be capable of air quenching in lieu of liquid quenching to further enhance brightness by eliminating staining.

In accordance with the present invention there have been developed new alloys which combine high strength properties and good surface extrusion and with the use of air quench methods, such as forced air circulation quench media. The new alloys of this invention are also capable of extrusion at higher rates of speed than the previously known alloys of equal mechanical strengths.

It has long been the practice to heat treat extrusions of heat-treatable aluminum alloys by a process known as solution heat treatment, to reach T-4 temper. Such treatment involves heating the extrusion to a temperature at which solution and diffusion of the heat-treatable alloying constituents takes place and produces, as nearly as practicable, a homogenous solid solution. The alloy then is quenched (i.e., rapidly cooled) in order to prevent the hardening constituents from precipitating substantially from solid solution during the cooling period. Slow cooling, on the other hand, would permit these constituents to precipitate to a greater extent, so that the alloy would be in a partially annealed condition unsuitable for subsequent precipitation heat treatment. Solution heat treatment, including the quench, is a necessary preliminary to subsequent precipitation heat treatment to T-6 temper, to increase the mechanical properties of the alloy.

T-4 temper is defined as solution heat treated and naturally aged to a substantially stable condition. Such natural aging may take one or more days, but the extrusion can be precipitattion heat treated to T-6 temper at any stage of natural aging to stable T-6 temper. For the purposes of this application, therefore, T-4 temper refers to the solution heat treated condition after quenching, regardless of subsequent natural aging.

One conventional method of solution heat treatment of heat-treatable aluminum alloys is carried out by operations separate from the forming operations, i.e., after the extrusion operation has been completed, the extrusions are removed from the press, heated to solution heat treatment temperature in a molten salt bath or in an air furnace and, after being heated for the required time at such temperature for solution of the soluble constituents to take place, are removed and immediately quenched. Another conventional method of solution heat treating an extrusion is to extrude it completely at about a solution heat treatment temperature, and then cut oii the extrusion and immediately quench it in a tank of water adjacent the run-out table. Still another conventional method of solution heat treating an extrusion is to lead it through a water trough built into the run-out table, so that the extrusion can be quenched continuously as it comes from the press. Some alloys, such as 6061 and 6062, require liquid quenching to obtain their potential high mechanical properties, whereas others, such as 6063, can be quenched with air instead of liquid to obtain their potential mechanical properties. However, regardless of how it is quenched, alloy 6063 has substantially lower strength than alloys 6061 and 6062 when the latter are liquid quenched.

In general, the achievement of the combination of high strength and brightness and high electrical conductivity in the new alloys of the invention is brought about by the elimination or reduction to a minimum of the metallurgically effective concentrations of copper, chromium, and iron in order to minimize the concentration of insoluble constituents in the alloy. The proportion of the grain refining element titanium is also reduced, while the Zinc content is limited to a maximum of 0.25%.

The combination of mechanical strength and bright a surface in the new alloys of this invention is also achieved by careful control of the amount of magnesium silicide that is in solid solution at the solution heat treat temperature range normally used for this type alloy (6001050 F.; preferably 950980 F.). This result is achieved by the presence in the alloys of magnesium and silicon in stoichiometric proportion to form the intermetallic compound Mg Si, plus a slight excess of silicon.

In accordance with this invention, the broad range of silicon content is from 0.40 to 0.80%, and the preferred range is from 0.50 to 0.70%. The broad range of magnesium content is from 0.70 to 1.10%, and the preferred range is from 0.80 to 1.00%. These ranges are consistent with the desired slight excess of silicon over that in stoichiometric proportion to the magnesium.

In the new alloys zinc is held at a maximum of 0.25%, preferably 0.15%, in which ranges zinc has little or no effect on the alloy. An adequate grain refining effect is obtainable with a maximum of 0.10% titanium, preferably 0.05%, but the titanium can be, and preferably is, omitted. Copper is held to a maximum of 0.20%, preferably 0.07 to 0.10%, primarily for control of localized pitting and other types of corrosion. Iron is capable of forming a separate constituent in alloys of aluminum with silicon, all or some of which are insoluble and contribute to poor surface quality; in the new alloys iron is held to a maximum of 0.50%, preferably at 0.20 to 0.40%. In the alloys of this invention, stress corrosion if present, can be overcome by the presence of as little as 0.10% maximum chromium, and preferably as little as 0.05%. The limit of 0.10% maximum of manganese, is again desirable to help control the amount of insoluble constituents and should only be intentionally added where specific conditions indicate its need, such as more resistance to corrosion and better aging characteristics. To achieve maximum electrical conductivity of the new alloys, the titanium and vanadium should each be less than 0.003%.

The marked change in the amounts of copper, iron, manganese, titanium, and zinc, together with the presence of magnesium and of silicon in slight stoichiometric excess to form magnesium silicide, results in retarding the precipitation rate of the soluble constituents sulficiently at the extrusion press when the alloys are extruded utilizing air quench method, to permit attainment of high tensile strength, high yield strength, and good surface properties. The change in precipitation rate at the extrusion press permits greater facility in achieving shapes, with more accurate tolerances (because of the less severe air quench) and higher speeds of extrusion. The rate of precipitation of the new alloys is similar to that of alloy 6063, but the resulting physical and extrusion characteristics are superior. The new alloys can be extruded at higher speeds than is the case with alloys 6061 and 6062.

In respect to nominal chemical composition, the alloys of this invention fall within the broad and preferred ranges shown in the following table:

TABLE 2 Composition of Heat T reatable Alloys In the alloys set forth in the foregoing table, the percentage of magnesium silicide present will range from about 1% to about 1.60%, preferably about 1.42%.

The composition of the novel alloys of the present invention differs from that of the known alloys 6061, 6062 and 6063 in the permissible maxirna for zinc, iron and copper, and in that titanium can be omitted. Moreover, the known alloys, such as 6061 and 6062 require liquid quenching conventionally after reheating to solution heat treatment temperature after extrusion to attain full T-4 and T-6 properties. Such separate heat treatment constitutes standard industry practice for alloys of the type of 6061, 6062 and 6063 (optional for 6063), as indicated, for example, for Alloys 6061, 6062 and 6063 (in Alloys Digest, Filing Code A1-3, Al-ll and Al-42, respectively, published by Engineering Alloys Digest, Inc., Upper Montclair, N.J.).

The novel alloys of this invention can be air quenched as they emerge from an extrusion press after being brought to solution heat treatment temperature prior to extrusion, with the result that a bright finish is obtained but the mechanical strength is nevertheless relatively high. In the case of alloys 6061 and 6062, full mechanical strength cannot be developed by air quenching; instead, it is necessary to bring these alloys to solution heat treatment temperature and then water quench them to T-4 temper. When alloys 6061 and 6062 are water quenched, their mechanical strength is relatively high but their surface brightness is relatively low. On the other hand, alloy 6063 is capable of being air quenched at the press, in which case it has excellent surface brightness, similar to that of the alloys of the invention, but the mechanical strength of alloy 6063 is substantially less than that of the new alloys of the invention when air quenched at the press, regardless of how alloy 6063 is heat treated and quenched. It can thus be seen that the new alloys of the invention combine the good properties of the air quenching capability and surface brightness of alloy 6063, with the good mechanical strength properties of alloys 6061 and 6062. In addition, the alloys of the invention have high electrical conductivity.

The heat treatable wrought aluminum alloys of the present invention are capable of heat treament to develop their full mechanical properties. The heat treatment may consist of high temperature solution heat treatment at between for example, about 900 and about 1000 F., followed by air or water quenching, to obtain T-4 temper. If it is desired to obtain full T-6 temper, the alloys may be subjected to solution heat treatment as indicated, followed by a precipitation or aging treatment at elevated temperatures ranging from about 300 to about 500 F., preferably about 350 F., for 4 to 8 hours. Moreover, the solution heat treatment to obtain T-'4 temper may be carried out by preheating a billet of the alloy to solution heat treatment temperature, extiuding it, and air quenching it to T-4 temper as it emerges from the press at solution heat treatment temperature, without the necessity of a separate solution heat treatment to bring the alloy to that temper after the extrusion of the alloy has been removed from the extrusion press. The time of the subsequent precipitation or aging treatment from the T-6 temper may vary from about /2 to 24 hours or more, depending on the temperature used, the alloy, and the properties desired. The solution heat treatment must include maintaining the alloy at the required temperature range for solution of the soluble constituents to take place, particularly that of magnesium silicide. The practical range of such temperatures may fall betwen about 800 and 1050 F. The precipitation or aging treatment may be carried out by conventional means, preferably at a temperature between about 345 F. and 355 F., for about 6 hours.

Mechanical strength testing of the new alloys for tensile, yield and elongation characteristics was carried out in accordance with the methods set forth in Federal Test Methods Standard No. 151, Supplement A. Preparation of test specimens was in accordance with Section 4 of Method 211.1 headed Tension Test in the aforesaid Standard 151. The 0.2% offset method was used to determine yield strength in accordance with paragraph 5.7.1 and FIGURE 13 of Method 211.1. Tensile strength was tested according to paragraph 5.11 of Method 211.1 and the elongation (percent in 2 inch gauge length) acacording to paragraph 5.12 of the Method 211.1. These methods are commonly used in the aluminum industry for production and research testing.

The new alloys of this invention exhibit a minimum tensile strength of about 37,000 pounds per sq. in., and a yield strength of at least 34,000 p.s.i. In thicknesses of 0.124 inch or less, the minimum elongation is 8%, while in thicknesses of 0.125 inch or more, the elongation is Typical T6 mechanical properties in the new alloys are a tensile strength of 41,000 p.s.i., and a yield strength of 38,000 p.s.i., and elongation of 12%, as compared with 35,000 p.s.i., 31,000 p.s.i., and 12%, respectively for alloy 6063-T6.

Thus, the new alloys attain substantially greater strengths than are possessed by allow 6063, as demonstrated by the data in Tables 3 and 4 below, showing T-4 tensile strengths ranging from 24,300 p.s.i. to as high as 39,400 p.s.i., as contrasted with only 25,000 p.s.i. typically listed commercially for alloy 6063. Similarly, the T6 tensile strengths in Tables 3 and 4 are shown in the range of 38,000 to 48,200 p.s.i., as compared with 35,000 p.s.i. tensile strength typically listed commercially for alloy 6063. Alloy 6063, moreover, is not capable of attaining this high tensile strength even when combined with the solution heat treatment contemplated by the present novel process.

By reason of their unexpected high electrical conductivity properties, the new alloys of this invention are especially adapted for uses in the electrical field, such as, for example, bus bars. It is known to employ alumihum of relatively high purity (at least 99.45% aluminum), designated EC. (electrical conductor) grade, for bus bar applications. However, owing to the relatively poor mechanical properties of this alloy, another aluminum alloy was developed, designated alloy 6101, containing 0.30-0.7 silicon, 0.35-08 magnesium, maximum limits of 0.10 copper, 0.03 manganese, 0.03 chromium, 0.10 Zinc, 0.06 iron, others each 0.03, others total 0.10, and the balance aluminum. Alloy 6101 in the T6 temper has mechanical properties comparable to those of copper bus bars, with a slight sacrifice in electrical conductivity as compared to EC, and is closely similar to alloy 6063 in composition, mechanical properties and response to heat treatment, surface brightness and electrical conductivity, but by special aging after quenching the electrical conductivity of alloy 6101 can be improved over 6063, at the expense of diminished mechanical properties. Alloys 6061 and 6062 in the T6 temper, while stronger than alloy 6101, have substantially lower electrical conductivity than alloy 6101.

In accordance with the present invention, it was discovered, in testing the novel alloys of this invention for electrical conductivity, that in the T6 temper they possessed a very high percent conductivity as compared with other alloys having the same mechanical strength properties, such as alloys 6061 and 6062. This high mechanical strength together with high electrical conductivity provides for the electrical industry an aluminum bus bar material sufiiciently hard to permit joining to other con necting bars or links by means of tight bolt connections while attaining sufficiently tight surface contact for maximum passage of electric current from one bar to the other, whereas the threading in the previously known softer Ibars would stnip upon bolting together. In -addi tion, the bus bars made of the new alloys of this invention possess ability to be plated with silver, corrosion resistance, and thermal expansion properties comparable to bus bars made of alloys 6101 and BC.

In measuring the percent electrical conductivity there is employed in this discussion the unit known as I.A.C.S., which is defined as the percent electrical conductivity of the International Annealed Copper Standard. Measurements are made in accordance with the Standiard Method of Tests for Resistivity of Electrical Conductor Materials, American Society for Testing Materials designation 13193-57.

Typical electrical conductivity values, together with mechanical properties, of the alloys of this invention in the T6 temper, are shown in the following table:

Typical electrical conductivity values, as related to the chemical composition of seven samples of the novel alloys of this invention, are set forth in the following table:

TABLE 2-B Composition Percent Sample Elec- 0. trieal 81 Fe Cu Mn Mg Cr Zn Ti Al Conductivity .29 0S 01 .88 00 .00 .00 Bal. 51. 6 .30 09 .01 .89 .00 .00 .00 Bal. 52. 5 .28 .08 .01 .90 .00 .00 .00 Bal. 52. 0 .20 .09 .01 .87 .00 .00 .00 Bal. 52.4 .28 .09 .01 .86 .00 .00 .00 Bal. 51.8 .28 07 .01 .91 .00 0O .00 Bai. 50.0 .29 .09 .01 .91 .00 .00 .00 Bal. 51.3

Average.

A further comparison of the mechanical and conductivity properties of the new alloys of this invention with those of known alloys, is given in Table 2-C:

TABLE 2-C Typical Mechanical Properties 1 Percent Al Alloys and Temper Conductivity Tensile Yield Percent Strength, Strength, Elongap.s.i. p.s.i. tion in 2 62 12,000 4, 000 56 82,000 28,000 15. 0 40 45, 000 40, 000 12.0 40 45,000 40, 000 12.0 6063T6 53 35,000 31,000 12.0 Typical new Alloy-T6 52 41, 000 38, 000 12. 0

The properties for the five conventional alloys are as listed in the Third Revision of Standard for Wrought Aluminum Mill Products, issued by the Aluminum Association, 420 Lexington Avenue, New York 17, N.Y.; and the properties for the new alloy are as published (alloy X6162), although higher values have since been found to be typical.

Table 2-C clearly shows the combination of high conductivity and high mechanical strength of the present alloys, as indicated by an average of the alloys of Table 2-B, as compared with the corresponding properties of other known alloys, and indicates the superiority of the new a'lloys.

This invention is illustrated by the following examples, but is not to be considered as limited thereto:

EXAM PLE 1 An alloy billet was prepared having the following nominal chemical composition:

Percent Silicon 0.61 Magnesium 1.05 Iron 0.20 Copper 0.07 Manganese 0.01 Chromium 0.00 Titanium 0.00 Zinc 0.00 Aluminum Balance The dimensions of the billet were 8 x 22.5". Prior to extrusion the previously homogenized billet was reheated to between 900 and 1000 F. to permit solid diffusion of the soluble constituents to take place. The billet discharged from. the reheating furnace at a temperature of about 1000 F. was fluid (Water) quenched H to a minimum temperature of about 850 F. and immediately extruded to approximately 50-foot length in hollow rectangular shape having ,a wall thickness of /s, in a single push of the extrusion press with forced air blast on the shape while being extruded. The ultimate front and back 4 feet of the extrusion were cut off, and five l-foot samples were cut from the front and back of the extrusion. The first 4 samples were furnace solution heat treated at temperatures of 930, 950, 970, and 990 F., and the fifth held in press-quenched condition, respectively.

The samples in each solution heat treatment group were divided, and one half were aged for 6 hours at 350 F. to the T-6 temper, then tested for tensile and yield strength and percent elongation. The other half of the group were also tested in their T-4 temper condition. The properties of the various samples exhibited the ranges shown in the following Table 5:

TABLE 3 Solution Heat Trcat- Tensile Yield Percent ment Temperature Temper Strength, Strength, Elongation 1,000 p.s.i. 1,000 p.s.i. in 2 25. 3-27. 14. -15. 6 19-23 27. 3-27. h 15. 0-17. 4 19-22 30. 3-30. 7 1s. 1l8. 3 21-23 33.1-33. 5 17. 3-21. 5 22-24 31. 13-33. 6 17. o-20. 9 21-22 38. n-as. s 34. 4-37. 0 11. o-12. 5 38. 7-39. 4 35. 8-37. 2 11. 5-12. 5 41. 0-41. 5 36. s-ss. 3 11. 5-13. 0 42. 0-12. 4 39. 3-39. 9 11. 5-13. 0 Press Quench-.- 41. 4-43. 4 37. 2-39. 2 10. 0-12. 4

EXAhIPLE 2 A billet having the nominal composition:

Percent Silicon 0.74 Magnesium 1.05 Iron 0.19 Copper 0.01 Magnesium 0.00 Chromium 0.00 Zinc 0.00 Titanium 0.00 Aluminum balance was extruded into furniture tubing of 1" outside diameter with a wall thickness of 0.050", following solution heat treatment at press temperature. The extrusion was air quenched at the press. Samples were taken of 8 the tubing without aging (T-4 temper), with short aging for 3 hours at 325355 F. (T-62), and with standard agin (T-6) for 6 hours at 345-355 F. The results are summarized in Table 4:

While present preferred embodiments of the invention, and methods of practicing the same, have been illustrated and described, it will be recognized that the invention may be otherwise variously embodied and practiccd within the scope of the following claims.

What is claimed is:

l. A wrought aluminum base alloy containing magnesium and silicon as essential components, characterized by a high mechanical strength and brightness of surface upon extrusion, and containing from 0.40% to 0.80% silicon, 0.70%-l.10% magnesium, with a maximum of 0.50% iron, 0.20% copper, 0.10% manganese, 0.10% chromium, 0.10% titanium, 0.25% zinc, 0.05% each of other elements, and 0.15% total of other elements, the balance being aluminum, said alloy containing silicon in slight excess over that required to combine stoichiometrically with the magnesium present to form magnesium silicide, said alloy being in the heat treated state produced by solution heat treatment at a temperature between about 900 and about 1000 F. followed by fluid quenching and aging.

2. The process which comprises solution heat treating a wrought aluminum base alloy containing magnesium and silicon as essential components, characterized by a high mechanical strength and brightness of surface upon extrusion, and containing from 0.40% to 0.80% silicon, 0.70%1.10% magnesium, with a maximum of 0.50% iron, 0.20% copper, 0.10% manganese, 0.10% chromium, 0.10% titanium, 0.25% zinc, 0.05% each of other elements, and 0.15% total of other elements, the balance being aluminum, said alloy containing silicon in slight excess over that required to combine stoichiometrically with the magnesium present to form magnesium silicide, at a temperature between about 900 F. and about 1000 F. and fiuid quenching said alloy.

3. The process which comprises solution heat treating a wrought aluminum base alloy containing magnesium and silicon as essential components, characterized by high mechanical strength and brightness of surface upon extrusion, and containing from 0.40% to 0.80% silicon, 0.70%-1.10% magnesium, with a maximum of 0.50% iron, 0.20% copper, 0.10% manganese, 0.10% chromium, 0.10% titanium, 0.25% zinc, 0.05% each of other elements, and 0.15% total of other elements, the

balance being aluminum, said alloy containing silicon in slight excess over that required to combine stoichiomctrically with the magnesium present to form magnesium silicide, at a temperature between about 900 F. and about 1000 F., fluid quenching and then aging said alloy at a temperature between about 300 F. and about 500 F. for about A2 to 24 hours.

References Cited in the file of this patent A loy Digest, Filing code A1-42, August 1956, published by Engineering Alloys Digest, Inc., Uppcr Montclair, New Jersey. 

2. THE PROCESS WHICH COMPRISES SOLUTION HEAT TREATING A WROUGHT ALUMINUM BASE ALLOY CONTAINING MAGNESIUM AND SILICON AS ESSENTIAL COMPONENTS, CHARACTERIZED BY A HIGH MECHANICAL STRENGTH AND BRIGHTENS OR SURFACE UPON EXTRUSION, AND CONTAINING FROM 0.40% TO 0.80% SILICON, 0.70%-1.10% MAGNESIUM, WITH A MAXIMUM OF 0.50% IRON 0.20% COPPER, 0.10% MANGANESE, 0.10% CHROMIUM, 0.10% RITANIUM, 25% ZINC 0.05% EACH OF OTHER ELEMENTS, AND 0.15% TOTAL OF OTHER ELEMENTS, THE BALANCE BEING ALUMINUM, SAID ALLOY CONTAINING SILICON IN SLIGHT EXCESS OVER THAT REQUIRED TO COMBINE STOICHIOMETRICALLY WITH THE MAGNESIUM PRESENT TO FORM MAGNESIUM SILICIDE, AT A TEMPERATURE BETWEEN ABOUT 900*F. AND ABOUT 1000* F. AND FLUID QUENCHING SAID ALLOY. 